Not applicable.
Not applicable.
This invention relates generally to the separation of oxygen from gas mixtures, such as air. More particularly, the invention relates to the use of mixed adsorbents in the adsorber, a higher air feed temperature, and/or a simpler vacuum swing adsorption/pressure swing adsorption (VSA/PSA) process design to separate oxygen from air.
In numerous chemical processing, refinery, metal production and other industrial applications, purified gas streams are employed for a variety of processing purposes. For example, high purity oxygen is used in chemical processing, steel mills, paper mills, and in lead and gas production operations. Oxygen and nitrogen are produced from air, typically by cryogenic distillation for large size applications. While such cryogenic processing can be very efficient, particularly when conducted in large size plants, it nevertheless requires complex and costly equipment.
Vacuum swing adsorption/pressure swing adsorption processes have also been used to separate and purify gases, but the production of oxygen by the VSA/PSA approach has generally been confined to relatively small-sized operations with respect to which the use of cryogenic air separation may not be economically feasible. Many commonly available adsorbents, particularly the class of materials known as molecular sieves or zeolites, selectively adsorb nitrogen more strongly than oxygen, and this preferential adsorption is the basis of a variety of VSA/PSA processes that have been developed for the separation of air to produce oxygen and nitrogen product gas.
In the VSA/PSA process, a feed gas mixture, such as air, containing a more readily adsorbable component and a less readily adsorbable component, e.g., the nitrogen and oxygen components of air, is passed to the feed end of an adsorbent bed capable of selectively adsorbing the more readily adsorbable component at a higher adsorption pressure. Most of the less readily adsorbable component passes through the bed and is recovered from the discharge end of the bed. Thereafter, the bed is depressurized to a lower desorption pressure for desorption of the more readily adsorbable component, and its removal from the feed end of the bed prior to repressurization with feed gas and or less readily adsorbed component, and introduction of fresh feed gas for adsorption as cyclic adsorption-desorption-repressurization operations are continued in the bed. Such VSA/PSA processing is commonly carried out in multi-bed systems, with each bed employing the same VSA/PSA processing sequence on a cyclic basis interrelated to the carrying out of such processing sequence in the other beds of the adsorption system. In VSA/PSA systems for the recovery of moderate to high purity oxygen (80-95% O2) product as the less readily adsorbable component of air, each adsorbent bed will commonly contain an adsorbent material capable of selectively adsorbing nitrogen as the more readily adsorbable component, with the selectively adsorbed nitrogen being subsequently desorbed and removed from the bed upon reduction of the pressure of the bed from the higher adsorption pressure level to a lower desorption pressure level. VSA/PSA systems for the recovery of nitrogen product have likewise been based on the use of adsorbents that selectively adsorb nitrogen from air as the more readily adsorbable component thereof.
There are various techniques that exist to separate nitrogen from oxygen. For instance, U.S. Pat. No. 4,329,158 to Sircar discloses a process for the separation of nitrogen from oxygen wherein a pretreatment adsorptive separation of water and carbon dioxide is performed prior to the bulk separation of the major constituents of air. Nitrogen enriched waste gas is utilized from the bulk separation portion of the process to regenerate the pretreatment portion of the process. The bulk separation of nitrogen from oxygen is performed with an elevated temperature adsorption of nitrogen, a desorption of bulk separation beds to a lower pressure, a purge of the beds with product oxygen after desorption countercurrently and two steps of repressurization to elevated pressure first with waste gas which is nitrogen enriched and secondly with product oxygen.
U.S. Pat. No. 5,882,380 to Sircar describes a single-bed PSA system comprising a blower, an adsorber vessel, and a gas product storage tank that separates a gas mixture using a three-step cycle comprising adsorption, evacuation, and pressurization used to separate nitrogen from a feed air. Pressurization is accomplished by introducing gas from the gas product storage tank into both the feed end and the product end of the adsorber vessel. Preferably a portion of the pressurization gas is introduced into the adsorber vessel by the blower, which also is used for providing feed to the adsorber and for withdrawing gas from the adsorber during the evacuation step.
Ackley et al. (European Patent Application No. 0 963 777) discloses a PSA apparatus for the separation of a heavy component from a light component in a feed stream. The apparatus includes an adsorbent bed comprising either a mixture of adsorbents or composite adsorbent particles wherein each particle comprises two or more adsorbents. At least one of the adsorbents is comparatively weak, i.e., NaX, and the other is comparatively strong, i.e., LiX. Another embodiment of the invention is a PSA prepurifier having a bed of adsorbent material which comprises a mixture of adsorbents, or composite of adsorbent particles wherein each particle comprises at least two adsorbents, at least one of the adsorbents being comparatively strong, i.e., NaY and at least another of the adsorbents being comparatively weak, i.e., activated aluminum.
The adsorbent is often the key to the effectiveness of oxygen production processes. Therefore, much attention has been given to the development, improvement and manufacture of adsorbents. For example, specialized zeolite adsorbents have been synthesized through ion exchange, lower Si/Al structures and improved activation procedures. These additional and/or improved manufacturing steps have resulted in higher costs for these specialized adsorbents (e.g., LiX) compared to more common adsorbents (e.g., 5A and 13X). In many processes, the adsorbent has become a significant fraction of the overall capital investment. Thus, there is considerable incentive to reduce the cost of the adsorbent if doing so results in an overall reduction in the cost of the desired product of the separation.
Accordingly, there is a need for alternative systems for separating oxygen from gas mixtures, such as air, wherein the systems optimize the productivity of relatively inexpensive adsorbents to render the systems economically competitive with state of the art systems employing more sophisticated but expensive adsorbents.
All references cited herein are incorporated herein by reference in their entireties.
The invention provides a process for producing an oxygen enriched product from a feed gas containing oxygen and nitrogen. The process comprises: (a) providing a gas separation apparatus having at least one bed containing a physical mixture of at least two different nitrogen selective adsorbents, wherein the at least one bed is free of lithium cations; (b) feeding a feed gas containing oxygen and nitrogen into the gas separation apparatus to contact the at least one bed; and (c) recovering from the gas separation apparatus the oxygen enriched product. The process is preferably performed above ambient temperature and/or in a simplified four-step VSA/PSA cycle. The cycle includes: (a) feeding a feed gas containing oxygen and nitrogen into a gas separation apparatus to contact at least one bed of the apparatus with the feed gas, wherein the feed gas is at a temperature above ambient, e.g., from about 40xc2x0 C. to about 100xc2x0 C.; (b) countercurrently evacuating the at least one bed following the feeding; (c) countercurrently purging the at least one bed with the oxygen enriched product under vacuum; and (d) simultaneously pressurizing the at least one bed with a countercurrent stream of the oxygen enriched product and a cocurrent stream of the feed gas.
Also provided are an apparatus for performing the process of the invention and a process for producing an oxygen enriched product from a feed gas containing oxygen and nitrogen using the four-step cycle and elevated temperature with lithium-containing and/or lithium-free beds.